As is well known in the art, high pressure sodium (HPS) lamps are difficult to start and require special circuitry for restarting if the lamp is extinguished after sufficient operation to elevate its temperature. This is normally known as hot restarting and is known to require high voltage and energy across the lamp, considerably higher than can be provided by the line operating voltage.
In commonly-assigned U.S. Pat. Nos. 5,047,694 and 5,321,338, various hot restarting circuits for HPS and other high intensity discharge lamps are described. These circuits include a storage capacitor and an SCR connected across a tapped portion of a ballast with a breakdown device to start the SCR. A charging circuit for the storage capacitor includes a diode, a pumping capacitor and a choke connected in series from the ballast tap to the AC line, and a further diode interconnecting the capacitors. The pumping capacitor increases the charge on the storage capacitor in a stepwise fashion until the breakdown voltage is reached, whereupon energy in the form of high voltage starting pulses is applied to the lamp.
In cases where a high intensity discharge lamp is defective or is otherwise incapable of starting, it is desirable to automatically disable the starting circuit after a certain period of time in order to prevent damage to the dielectric components of the circuit from repeated high voltage pulses. The aforementioned U.S. Pat. Nos. 5,047,694 and 5,321,338 disclose two ways in which this may be accomplished. In one embodiment, a thermostatic switch is connected in series between the pumping capacitor and storage capacitor and is opened by an associated heating resistor after a certain period of time (approximately 3 to 5 minutes) to terminate the stepwise charging of the storage capacitor. Although this is an effective arrangement, it has the disadvantage that the disablement time will depend to some extent on the ambient temperature at the luminaire, which can range from -30.degree. C. for an outdoor installation to more than +90.degree. C. when the HPS lamp is operating. In a second embodiment, which does not have this disadvantage, the disabling circuit is electronic in operation rather than thermal. In this embodiment, a capacitor having a value much larger than that of the storage capacitor is used to slowly accumulate a charge that opposes the charge on the storage capacitor, eventually preventing the storage capacitor from attaining the necessary breakdown voltage. In the specific embodiment disclosed, the high voltage starting pulses are generated every 0.45 second and are terminated by the disabling circuit after 4 pulses have occurred. Despite its temperature insensitivity, however, the capacitive disabling circuit is disadvantageous in that it requires a high value capacitor (on the order of 100 microfarads) which is not only expensive, but is physically large and difficult to fit onto the same circuit board with other HPS starting components.